Tobacco (Nicotiana tabacum L.) is an important commercial crop in the United States as well as in other countries. Blue mold is one of the most significant foliar diseases of tobacco. When weather conditions are favorable, the disease spreads rapidly and attacks plants throughout the growing season. It can completely destroy transplants in the bed. In the field, the presence of the pathogen can be seen as brown necrotic spots on the leaves or as a systemic infection.
Control of the pathogen can be achieved by two means: the use of fungicides and the introduction of resistant varieties. The development of fungicide resistant strains of the fungus has increased the need for resistant varieties. Naturally occurring host resistance to Peronospora tabacina exists among wild Nicotiana species mainly of Australian origin, where the pathogen is endemic. Transfer of resistance into cultivated tobacco from various sources has been successfully achieved via interspecific hybridization. The most widely used sources are N. debneyi accessions. Commercially grown burley cultivars are either susceptible or very susceptible to the disease, with the exception of TN 90, which is relatively tolerant, but is not resistant.
Accordingly, it would be desirable to provide a tobacco cultivar that demonstrates blue mold resistance.
There are numerous stages in the development of any novel, desirable plant germplasm. Plant breeding begins with the analysis and definition of problems and weaknesses of the current germplasm, the establishment of program goals, and the definition of specific breeding objectives. The next step is selection of germplasm that possess the traits to meet the program goals. The aim is to combine in a single variety an improved combination of desirable traits from the parental germplasm. These important traits may include higher yield, resistance to diseases and insects, better stems and roots, tolerance to drought and heat, improved nutritional quality, and better agronomic quality.
Choice of breeding or selection methods depends on the mode of plant reproduction, the heritability of the trait(s) being improved, and the type of cultivar used commercially (e.g., F1 hybrid cultivar, pureline cultivar, etc.). For highly heritable traits, a choice of superior individual plants evaluated at a single location may be effective, whereas for traits with low heritability, selection should be based on mean values obtained from replicated evaluations of families of related plants. Popular selection methods commonly include pedigree selection, modified pedigree selection, mass selection, and recurrent selection.
The complexity of inheritance influences the choice of breeding method. Backcross breeding is used to transfer one or a few favorable genes for a highly heritable trait into a desirable cultivar. This approach has been used extensively for breeding disease-resistant cultivars. Various recurrent selection techniques are used to improve quantitatively inherited traits controlled by numerous genes. The use of recurrent selection in self-pollinating crops depends on the ease of pollination, the frequency of successful hybrids from each pollination, and the number of hybrid offspring from each successful cross.
Each breeding program should include a periodic, objective evaluation of the efficiency of the breeding procedure. Evaluation criteria vary depending on the goals and objectives, but should include gain from selection per year based on comparisons to an appropriate standard, overall value of the advanced breeding lines, and number of successful cultivars produced per unit of input (e.g., per year, per dollar expended, etc.).
Promising advanced breeding lines are typically tested and compared to appropriate standards in environments representative of the commercial target area(s) for three or more years. The best lines are candidates for new commercial cultivars; those still deficient in a few traits may be used as parents to produce new populations for further selection.
An important task is the identification of individuals that are genetically superior, because for most traits the true genotypic value is masked by other confounding plant traits or environmental factors. One method of identifying a superior plant is to observe its performance relative to other experimental plants and to a widely grown standard cultivar. If a single observation is inconclusive, replicated observations provide a better estimate of its genetic worth.
The goal of a tobacco breeding program is to develop new, unique and superior tobacco cultivars and hybrids. The breeder typically initially selects and crosses two or more parental lines, followed by repeated selfing and selection, producing many new genetic combinations. In tobacco, completely homozygous doubled-haploid plants may also be generated (Burk et al., (1979) Science 206:585). The breeder can theoretically generate billions of different genetic combinations via crossing, selfing and mutations.
Each year, the plant breeder selects the germplasm to advance to the next generation. This germplasm is grown under different geographical, climate and soil conditions, and further selections are then made, both during and at the end of the growing season. The cultivars which are developed are unpredictable. This unpredictability is because the breeder's selection occurs in unique environments and there are millions of different possible genetic combinations being generated. A breeder of ordinary skill in the art cannot predict the final resulting lines, except in a very general fashion. The same breeder cannot produce the same cultivar twice by using the exact same original parents and the same selection techniques. This unpredictability results in the expenditure of large amounts of research monies to develop superior new tobacco cultivars.
The development of new tobacco hybrids involves the development and selection of tobacco breeding lines, the crossing of these breeding lines and selection of superior hybrid crosses. The hybrid seed is produced by manual crosses between selected male-fertile parents or by using male sterility systems. Hybrid combinations are identified and developed on the basis of certain single gene traits such as leaf size or color, flower color, disease resistance or herbicide resistance, and the like, which are expressed in a hybrid. Additional data, such as yield and quality traits, on parental lines as well as the phenotype of the hybrid influence the breeder's decision to continue with the specific hybrid cross.
Pedigree breeding and recurrent selection breeding methods are used to develop true breeding cultivars from breeding populations. Breeding programs combine desirable traits from two or more cultivars or various broad-based sources into breeding pools from which cultivars are developed by selfing or alternatively, by creating doubled-haploids, and selection of desired phenotypes. The new cultivars are evaluated to determine which have commercial potential.
Pedigree breeding is commonly used for the improvement of self-pollinating crops and parental lines for hybrids. Two parents which possess favorable, complementary traits are crossed to produce an F1. An F2 population is produced by selfing one or several F1 plants. Selection of the best individuals may begin in the F2 population; then, beginning in the F3, the best individuals in the families are selected. Replicated testing of families can begin in the F4 generation to improve the effectiveness of selection for traits with low heritability. At an advanced stage of inbreeding (i.e., F6 and F7), the best lines or mixtures of phenotypically similar lines are tested for potential release as new cultivars.
Mass and recurrent selections can be used to improve populations of either self- or cross-pollinating crops. A genetically variable population of heterozygous individuals is either identified or created by intercrossing several different parents. The best plants are selected based on individual superiority, outstanding progeny, or excellent combining ability. The selected plants are intercrossed to produce a new population in which further cycles of selection are continued.
Backcross breeding has been used to transfer genes for a simply inherited, highly heritable trait into a desirable homozygous cultivar or inbred line which is the recurrent parent. The source of the trait to be transferred is called the donor parent. The resulting plant is expected to have the attributes of the recurrent parent (e.g., cultivar) and the desirable trait transferred from the donor parent. After the initial cross, individuals possessing the phenotype of the donor parent are selected and repeatedly crossed (backcrossed) to the recurrent parent. The resulting plant is expected to have the attributes of the recurrent parent (e.g., cultivar) and the desirable trait transferred from the donor parent.
The single-seed descent procedure in the strict sense refers to planting a segregating population, harvesting a sample of one seed per plant, and using the one-seed sample to plant the next generation. When the population has been advanced from the F2 to the desired level of inbreeding, the plants from which lines are derived will each trace to different F2, individuals. The number of plants in a population declines in each generation due to failure of some seeds to germinate or some plants to produce at least one seed. As a result, not all of the F2 plants originally sampled in the population will be represented by a progeny when generation advance is completed.
In a multiple-seed procedure, tobacco breeders harvest seeds from one or more flowers from each plant in a population and pool them to form a bulk. Part of the bulk is used to plant the next generation and part is put in reserve. The procedure has been referred to as modified single-seed descent technique.
Proper testing should detect any major faults and establish the level of superiority or improvement over current cultivars. In addition to showing superior performance, there must be a demand for a new cultivar that is compatible with industry standards or which creates a new market. The introduction of a new cultivar will incur additional costs to the seed producer, the grower, the processor and the consumer, for special advertising and marketing, altered seed and commercial production practices, and new product utilization. The testing preceding release of a new cultivar should take into consideration research and development costs as well as technical superiority of the final cultivar. For seed-propagated cultivars, it must be feasible to produce seed easily and economically.
Maternal haploids can be obtained by pollination of plants of N. tabacum with N. africana. Numerous seeds develop in fruits from this cross, but germinating interspecific hybrid seedlings are highly lethal (99.9%). Surviving F1 plants consist of mixtures of aneuploid interspecific hybrids and maternal haploids. The chromosomes of the maternal haploids are derived from the N. tabacum female plant. The procedure is very simple, but requires technical skill to distinguish phenotypically the aneuploid interspecific hybrids from maternal haploids in seedling stages. Environmental effects on tobacco females crossed with N. africana pollen greatly influence the number of haploids obtained per capsule. One to three haploid plants frequently can be obtained from a capsule of a tobacco× N. africana cross when the tobacco female is grown in the field. Haploid numbers per pollination of greenhouse-grown tobacco are five to ten times lower. Chromosome-doubled haploids obtained by this technique are superior to ADH lines from the same parental sources and more closely resemble the performance of conventionally developed inbred genotypes.
Methods of tobacco breeding are discussed in detail in Wernsman, E. A., and Rufty, R. C. 1987. Chapter Seventeen. Tobacco. Pages 669-698 In: Cultivar Development. Crop Species. W. H. Fehr (ed.), MacMillan Publishing Go., Inc., New York, N.Y. 761 pp.